In the bottom left corner you will see a box that says “Default – Personal workspace” (or something else if you already changed it).
Click that to reveal a new context menu.
Click “Manage” next to the workspace you want to add a user to.
Note: Your “Default – Personal workspace” cannot have users added to it.
Click “Members” and a new window will popup.
Type in the email address of the user you want to add to the workspace.
Be careful to select an appropriate role. Learn more about “Roles” here.
The user will now get an invite email to join the work space.